fix: improve stdin input handling

Improve how mmdc handles stdin inputs.

Currently, running `mmdc` exits with an error if it cannot find
an stdin. However, it does this by checking if the stdin is a FIFO,
which is only true in some cases.
(it seems to fail on Windows and other non-bash shells).

I've changed `mmdc` to always read from stdin if there is no `--input`.
However, it prints a warning.

To suppress this warning, you can run mmdc with `--input -`.

Author: aloisklink

README.md

@@ -109,7 +109,7 @@ You can easily pipe input from stdin. This example shows how to use a heredoc to
 send a diagram as stdin to mermaid-cli (mmdc).
 
 ```sh
-cat << EOF  | mmdc
+cat << EOF  | mmdc --input -
     graph TD
     A[Client] --> B[Load Balancer]
 EOF

src-test/test.js

@@ -1,6 +1,6 @@
 import fs from 'fs/promises'
 // Can't use async to load workflow entries, see https://github.com/facebook/jest/issues/2235
-import { readdirSync } from 'fs'
+import { readdirSync, createReadStream } from 'fs'
 import { exec, execFile } from 'child_process'
 
 // Joins together directory/file names in a OS independent way
@@ -12,6 +12,7 @@ import { expect, beforeAll, afterAll, describe, test } from '@jest/globals'
 
 import { run, renderMermaid, parseMMD } from '../src/index.js'
 import puppeteer from 'puppeteer'
+import { pipeline } from 'stream'
 
 const workflows = ['test-positive', 'test-negative']
 const out = 'test-output'
@@ -154,8 +155,24 @@ describe('mermaid-cli', () => {
     ).rejects.toThrow('TimeoutError: Timed out after 1 ms')
   }, timeout)
 
-  test('should error on missing input', async () => {
-    await expect(promisify(execFile)('node', ['src/cli.js'])).rejects.toThrow()
+  test('should warn when reading from stdin with missing --input', async () => {
+    const execFilePromise = promisify(execFile)('node', ['src/cli.js'])
+    await promisify(pipeline)(
+      createReadStream('test-positive/flowchart1.mmd'),
+      execFilePromise.child.stdin
+    )
+    const { stderr } = await execFilePromise
+    expect(stderr).toContain('No input file specfied, reading from stdin.')
+  }, timeout)
+
+  test('should not warn when reading from stdin with `--input -`', async () => {
+    const execFilePromise = promisify(execFile)('node', ['src/cli.js', '--input', '-'])
+    await promisify(pipeline)(
+      createReadStream('test-positive/flowchart1.mmd'),
+      execFilePromise.child.stdin
+    )
+    const { stderr } = await execFilePromise
+    expect(stderr).not.toContain('No input file specfied, reading from stdin.')
   }, timeout)
 
   test('should error on mermaid syntax error', async () => {

src/index.js

@@ -27,8 +27,6 @@ const checkConfigFile = file => {
   }
 }
 
-const inputPipedFromStdin = () => fs.fstatSync(0).isFIFO()
-
 const getInputData = async inputFile => new Promise((resolve, reject) => {
   // if an input file has been specified using '-i', it takes precedence over
   // piping from stdin
@@ -84,7 +82,7 @@ async function cli () {
     .addOption(new Option('-t, --theme [theme]', 'Theme of the chart').choices(['default', 'forest', 'dark', 'neutral']).default('default'))
     .addOption(new Option('-w, --width [width]', 'Width of the page').argParser(parseCommanderInt).default(800))
     .addOption(new Option('-H, --height [height]', 'Height of the page').argParser(parseCommanderInt).default(600))
-    .option('-i, --input <input>', 'Input mermaid file. Files ending in .md will be treated as Markdown and all charts (e.g. ```mermaid (...)```) will be extracted and generated. Required.')
+    .option('-i, --input <input>', 'Input mermaid file. Files ending in .md will be treated as Markdown and all charts (e.g. ```mermaid (...)```) will be extracted and generated. Use `-` to read from stdin.')
     .option('-o, --output [output]', 'Output file. It should be either md, svg, png or pdf. Optional. Default: input + ".svg"')
     .addOption(new Option('-e, --outputFormat [format]', 'Output format for the generated image.').choices(['svg', 'png', 'pdf']).default(null, 'Loaded from the output file extension'))
     .addOption(new Option('-b, --backgroundColor [backgroundColor]', 'Background color for pngs/svgs (not pdfs). Example: transparent, red, \'#F0F0F0\'.').default('white'))
@@ -101,12 +99,15 @@ async function cli () {
   let { theme, width, height, input, output, outputFormat, backgroundColor, configFile, cssFile, puppeteerConfigFile, scale, pdfFit, quiet } = options
 
   // check input file
-  if (!(input || inputPipedFromStdin())) {
-    console.error(chalk.red('\nPlease specify input file: -i <input>\n'))
-    // Log to stderr, and return with error exitCode
-    commander.help({ error: true })
-  }
-  if (input && !fs.existsSync(input)) {
+  if (!input) {
+    warn('No input file specfied, reading from stdin. ' +
+      'If you want to specify an input file, please use `-i <input>.` ' +
+      'You can use `-i -` to read from stdin and to suppress this warning.'
+    )
+  } else if (input === '-') {
+    // `--input -` means read from stdin, but suppress the above warning
+    input = undefined
+  } else if (!fs.existsSync(input)) {
     error(`Input file "${input}" doesn't exist`)
   }